6 mm remington

[sagela]

Correct me if I'm wrong but should I use a faster powder for a 6mm in a remington 600 with a 18" barrel all the data I see is for a 22" barrel

If you're looking for velocity, a faster powder might help. If you don't have a chronograph, note the impact point (given the same sight picture). If the slower powder prints higher
on the target, you're likely getting more velocity.
But when accuracy is the goal you just have to keep trying powders/charges/seating depths/primers, etc.